High temperature aluminium alloy

ABSTRACT

In an aluminium alloy of type AlMgSi with good creep strength at elevated temperatures for the production of castings subject to high thermal and mechanical stresses the contents of the alloying elements magnesium and silicon in % w/w in a Cartesian coordinate system are limited by a polygon A with the coordinates [Mg; Si] [8.5; 2.7] [8.5; 4.7] [6.3; 2.7] [6.3; 3.4] and that the alloy also contains
     0.1 to 1% w/w manganese   max. 1% w/w iron   max. 3% w/w copper   max. 2% w/w nickel   max. 0.5% w/w chromium   max. 0.6% w/w cobalt   max. 0.2% w/w zinc   max. 0.2% w/w titanium   max. 0.5% w/w zirconium   max. 0.008% w/w beryllium   max. 0.5% w/w vanadium
 
as well as aluminium remainder rest with further elements and manufacturing-related impurities of individually max. 0.05% w/w and max. 0.2% w/w in total.
   

     The alloy is suitable in particular for the production of cylinder crankcases by the pressure die casting method.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an aluminium alloy of type AlMgSi with good creep strength at elevated temperatures for the production of castings subject to high thermal and mechanical stresses.

2. Description of the Prior Art

The further development of diesel engines with the aim of achieving an improved combustion of the diesel fuel and a higher specific output leads inter alia to a higher explosion pressure and in consequence to a pulsating mechanical load acting on the cylinder crankcase that makes very high demands on the material. Apart from a high fatigue strength, a good endurance strength at high temperatures of the material is a further precondition for its use in the production of cylinder crankcases.

AlSi alloys are generally used today for components subject to high thermal stresses, this high-temperature strength being achieved by the addition of Cu to the alloy. Copper does, however, also increase the hot shortness and has a negative effect on the castability. Applications in which in particular high-temperature strength is demanded are primarily found in the area of the cylinder heads of automotive engines, see e.g. F. J. Feikus, “Optimierung von Aluminium-Silicium-Gusslegierungen für Zylinderköpfe” [Optimization of Aluminium-Silicon Casting Alloys for Cylinder Heads], Giesserei-Praxis, 1999, Volume 2, pp. 50-57.

A high-temperature AlMgSi alloy for the production of cylinder heads is known from U.S. Pat. No. 3,868,250. The alloy contains, apart from the normal additives, 0.6 to 4.5% w/w Si, 2.5 to 11% w/w Mg, of which 1 to 4.5% w/w free Mg, and 0.6 to 1.8% w/w Mn.

WO-A-96 15281 describes an aluminium alloy with 3.0 to 6.0% w/w Mg, 1.4 to 3.5% w/w Si, 0.5 to 2.0% w/w Mn, max. 0.15% w/w Fe, max. 0.2% w/w Ti and aluminium as remainder with further impurities of individually max. 0.02% w/w, and max. 0.2% w/w in total. The alloy is suitable for the production of components where high demands are made on the mechanical properties. Processing of the alloy is preferably by pressure die casting, thixocasting or thixoforging.

A similar aluminium alloy for the production of safety components by pressure die casting, squeeze casting, thixoforming or thixoforging is known from WO-A-0043560. The alloy contains 2.5-7.0% w/w Mg, 1.0-3.0% w/w Si, 0.3-0.49% w/w Mn, 0.1-0.3% w/w Cr, max. 0.15% w/w Ti, max. 0.15% w/w Ti, max. 0.15% w/w Fe, max. 0.00005% w/w Ca, max. 0.00005% w/w Na, max. 0.0002% w/w P, further impurities of individually max. 0.02% w/w and aluminium as remainder.

A casting alloy of type AlMgSi known from EP-A-1 234 893 contains 3.0 to 7.0% w/w Mg, 1.7 to 3.0% w/w Si, 0.2 to 0.48% w/w Mn, 0.15 to 0.35% w/w Fe, max. 0.2% w/w Ti, optionally also 0.1 to 0.4% w/w Ni and Al as remainder and manufacturing-related impurities of individually max. 0.02% w/w and max. 0.2% w/w in total, with the further condition that magnesium and silicon in the alloy essentially exist in a ratio Mg:Si of 1.7:1 by weight, corresponding to the composition of the quasi-binary eutectic with the solid phases Al and Mg₂Si. The alloy is suitable for the production of safety components in motor vehicles by pressure die casting, rheocasting and thixocasting.

The object of the invention is to provide an aluminium alloy with good creep strength at elevated temperatures for the production of components subject to high thermal and mechanical stresses. The alloy should be suitable in particular for pressure die casting, but also for gravity die casting, low-pressure die casting and sand casting.

A specific object of the invention is the provision of an aluminium alloy for cylinder crankcases of internal combustion engines, in particular of diesel engines, produced by pressure die casting.

The components cast from the alloy should exhibit high strength together with high ductility. The intended mechanical properties in the component are defined as follows:

Proof strength Rp0.2 > 170 MPa Tensile strength Rm > 230 MPa Elongation at break A5 > 6%

The castability of the alloy should be comparable with the castability of the AlSiCu casting alloys currently used, and the alloy should not show any tendency to hot shortness.

SUMMARY OF THE INVENTION

The object is achieved with the solution according to the invention in that the contents of the alloying elements magnesium and silicon in % w/w in a Cartesian coordinate system are limited by a polygon A with the coordinates [Mg; Si] [8.5; 2.7] [8.5; 4.7] [6.3; 2.7] [6.3; 3.4] and that the alloy also contains

0.1 to 1% w/w manganese max. 1% w/w iron max. 3% w/w copper max. 2% w/w nickel max. 0.5% w/w chromium max. 0.6% w/w cobalt max. 0.2% w/w zinc max. 0.2% w/w titanium max. 0.5% w/w zirconium max. 0.008% w/w beryllium max. 0.5% w/w vanadium as well as aluminium as remainder with further elements and manufacturing-related impurities of individually max. 0.05% w/w and max. 0.2% w/w in total.

The following content ranges are preferred for the main alloying elements, Mg and Si:

Mg 6.9 to 7.9% w/w, in particular 7.1 to 7.7% w/w Si 3.0 to 3.7% w/w, in particular 3.1 to 3.6% w/w

Particularly preferred are alloys whose contents of the alloying elements magnesium and silicon in % w/w in a Cartesian coordinate system are limited by a polygon B with the coordinates [Mg; Si] [7.9; 3.0] [7.9; 3.7] [6.9; 3.0] [6.9; 3.7], in particular by a polygon C with the coordinates [Mg; Si] [7.7; 3.1] [7.7; 3.6] [7.1; 3.1] [7.1; 3.6].

The alloying elements Mn and Fe allow sticking of the castings to the mould to be avoided. A higher iron content results in a higher high-temperature strength at the expense of reduced elongation. Mn contributes also significantly to red hardness. Depending on the field of application, the alloying elements Fe and Mn are therefore preferably balanced with one another as follows:

With a content of 0.4 to 1% w/w Fe, in particular 0.5 to 0.7% w/w Fe, a content of 0.1 to 0.5% w/w Mn, in particular 0.3 to 0.5% w/w Mn, is set. With a content of max. 0.2% w/w Fe, in particular max. 0.15% w/w Fe, a content of 0.5 to 1% w/w Mn, in particular 0.5 to 0.8% w/w Mn, is set.

The following content ranges are preferred for the further alloying elements:

Cu 0.2 to 1.2% w/w, preferably 0.3 to 0.8% w/w, in particular 0.4 to 0.6% w/w

Ni 0.8 to 1.2% w/w

Cr max. 0.2% w/w, preferably max. 0.05% w/w

Co 0.3 to 0.6% w/w Ti 0.05 to 0.15% w/w Fe max. 0.15% w/w Zr 0.1 to 0.4% w/w

Copper results in an additional increase in strength, but with increasing contents leads to a deterioration in the corrosion behaviour of the alloy.

The addition of cobalt allows the demoulding behaviour of the alloy to be further improved.

Titanium and zirconium improve the grain refinement. A good grain refinement contributes significantly to an improvement in the casting properties and mechanical properties.

Beryllium in combination with vanadium reduces the formation of dross. With an addition of 0.02 to 0.15% w/w V, preferably 0.02 to 0.08% w/w V, in particular 0.02 to 0.05% w/w V, less than 60 ppm Be are sufficient.

A preferred field of application of the aluminium alloy according to the invention is the production of components subject to high thermal and mechanical stresses by pressure die casting, mould casting or sand casting, in particular for cylinder crankcases for automotive engines produced by the pressure die casting method.

The alloy according to the invention also satisfies the mechanical properties demanded for structural components in automotive construction after a single-stage heat treatment without separate solution annealing.

BRIEF DESCRIPTION OF THE DRAWING

Further advantage, features and properties of the invention can be seen from the following description of preferred exemplary embodiments and from the drawing that shows in

FIG. 1 a diagram with the content limits for the alloying elements Mg and Si according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The polygon A shown in FIG. 1 defines the content range for the alloying elements Mg and Si, the polygons B and C refer to preferred ranges. The straight line E corresponds to the composition of the quasi-binary eutectic Al—Mg₂Si. The alloy compositions according to the invention thus lie on the side with an excess of magnesium.

The alloy according to the invention was cast into pressure die cast plates with different wall thicknesses. Tensile strength test specimens were manufactured from the pressure die cast plates. The mechanical properties proof strength (Rp0.2), tensile strength (Rm) and elongation at break (A) were determined on the tensile strength test specimens in the conditions

-   F As cast -   Water/F As cast, quenched in water after demoulding -   F>24 h As cast, >24 h storage at room temperature -   Water/F>24 As cast, quenched in water after demoulding, >24 h     storage at room temperature     and after various single-stage heat treatment processes at     temperatures in the range from 250° C. to 380° C. and after     long-term storage at temperatures in the range from 150° C. to 250°     C.

The alloys examined are summarized in Table 1. The letter A indicates alloys with copper additive, the letter B alloys without copper additive.

Table 2 shows the results of the mechanical properties determined on tensile strength test specimens of the alloys in Table 1.

An alloy not included in Tables 1 and 2 with good creep strength at elevated temperatures exhibited the following composition (in % w/w, wherein the expression “% w/w” used in the specification and claims means “% by weight”):

3.4 Si, 0.6 Fe, 0.42 Cu, 0.32 Mn, 7.4 Mg, 0.07 Ti, 0.9 Ni, 0.024 V and 0.004 Be

The results of the long-term tests underline the good creep strength at elevated temperatures of the alloy according to the invention. The mechanical properties after a single-stage heat treatment at 350° C. and 380° C. for 90 minutes indicate furthermore that the alloy according to the invention also satisfies the demands made for structural components in automotive construction.

TABLE 1 Chemical composition of the alloys in % w/w Wall thickness Alloy of flat variant specimen Si Fe Cu Mn Mg Ti V Be 1 3 mm 3.469 0.1138 0.787 7.396 0.106 0.0221 0.0025 1A 3 mm 3.4 0.117 0.527 0.781 7.151 0.119 0.0223 0.0019 2 2 mm 3.366 0.0936 0.774 7.246 0.117 0.0263 0.0024 2A 2 mm 3.251 0.0841 0.507 0.76 7.499 0.1 0.0246 0.0023 3 4 mm 3.352 0.0917 0.774 7.221 0.118 0.026 0.0024 3A 4 mm 3.198 0.0848 0.522 0.747 7.351 0.101 0.0255 0.0023 4 6 mm 3.28 0.0921 0.766 7.024 0.119 0.0268 0.0024 4A 6 mm 3.181 0.862 0.535 0.745 7.273 0.1 0.0257 0.0023

TABLE 2 Mechanical properties of the alloys Alloy Rp0.2 Rm A5 variant Initial state Heat treatment [MPa] [MPa] [%] 1 F 210 359 8.6 Water/F 181 347 9.6 F > 24 h 204 353 8.9 Water/F > 24 h 176 347 13.4 F > 24 h 250° C./10 min 216 352 7.4 250° C./20 min 218 352 6.8 250° C./90 min 207 349 10.8 350° C./10 min 154 315 12.5 350° C./20 min 158 315 10.6 350° C./90 min 147 306 11.4 380° C./10 min 145 304 14.1 380° C./20 min 139 299 13.9 380° C./90 min 137 299 16.7 150° C./100 h 221 365 9.4 180° C./100 h 214 346 6 200° C./100 h 211 354 9.4 250° C./100 h 184 336 11.7 150° C./500 h 223 353 6 180° C./500 h 216 357 9.7 200° C./500 h 202 349 9.2 250° C./500 h 170 327 12.3 1A F 234 345 4.2 Water/F 170 319 4.9 F > 24 h 205 355 7.1 Water/F > 24 h 188 340 5.6 F > 24 h 250° C./10 min 227 355 6.6 250° C./20 min 217 354 7.5 250° C./90 min 213 350 7.9 350° C./10 min 157 328 10.4 350° C./20 min 151 317 9.3 350° C./90 min 142 312 12.1 380° C./10 min 141 315 12.6 380° C./20 min 137 312 12.4 380° C./90 min 133 309 12.2 150° C./100 h 248 370 5 180° C./100 h 249 373 6.3 200° C./100 h 215 346 6.2 250° C./100 h 185 329 7.6 150° C./500 h 239 368 6.5 180° C./500 h 227 352 6.9 200° C./500 h 215 350 7.8 250° C./500 h 162 317 8.9 2 F > 24 h 212 364 10.7 250° C./90 min 223 358 9.9 350° C./90 min 152 312 13.9 380° C./90 min 139 297 17.9 2A F > 24 h 241 394 7.8 250° C./90 min 234 375 8.5 350° C./90 min 163 332 9 380° C./90 min 144 328 13.7 3 F > 24 h 158 321 9.9 250° C./90 min 164 324 10.4 350° C./90 min 143 307 12 380° C./90 min 129 292 16.4 3A F > 24 h 173 326 6 250° C./90 min 181 325 5.9 350° C./90 min 151 315 6.9 380° C./90 min 137 312 9.5 4 F > 24 h 138 304 8.2 250° C./90 min 145 309 9 350° C./90 min 133 297 8.4 380° C./90 min 123 286 12.7 4A F > 24 h 152 284 4.3 250° C./90 min 163 278 3.7 350° C./90 min 139 286 5.2 380° C./90 min 131 285 5.7 

1. Aluminium alloy of type AlMgSi with good creep strength at elevated temperatures for the production of castings subject to high thermal and mechanical stresses, characterized in that the contents of the alloying elements magnesium and silicon in % w/w in a Cartesian coordinate system are limited by a polygon A with the coordinates [Mg; Si] [8.5; 2.7] [8.5; 4.7] [6.3; 2.7] [6.3; 3.4] and that the alloy also contains 0.1 to 1% w/w manganese max. 1% w/w iron max. 3% w/w copper max. 2% w/w nickel max. 0.5% w/w chromium max. 0.6% w/w cobalt max. 0.2% w/w zinc max. 0.2% w/w titanium max. 0.5% w/w zirconium max. 0.008% w/w beryllium max. 0.5% w/w vanadium as well as aluminium as remainder with further elements and manufacturing-related impurities of individually max. 0.05% w/w and max. 0.2% w/w in total.
 2. Aluminium alloy according to claim 1, containing 6.9 to 7.9% w/w Mg.
 3. Aluminium alloy according to claim 1, containing 3.0 to 3.7% w/w Si.
 4. Aluminium alloy according to claim 1, characterized in that the contents of the alloying elements magnesium and silicon in % w/w in a Cartesian coordinate system are limited by a polygon B with the coordinates [Mg; Si] [7.9; 3.0] [7.9; 3.7] [6.9; 3.0] [6.9; 3.7].
 5. Aluminium alloy according to claim 4, characterized in that the contents of the alloying elements magnesium and silicon in % w/w in a Cartesian coordinate system are limited by a polygon C with the coordinates [Mg; Si] [7.7; 3.1] [7.7; 3.6] [7.1; 3.1] [7.1; 3.6].
 6. Aluminium alloy according to claim 1, containing 0.4 to 1% w/w Fe, and 0.1 to 0.5% w/w Mn.
 7. Aluminium alloy according to claim 1, containing max. 0.20% w/w Fe and 0.5 to 1% w/w Mn.
 8. Aluminium alloy according to claim 1, containing 0.2 to 1.2% w/w Cu.
 9. Aluminium alloy according to claim 1, containing 0.8 to 1.2% w/w Ni.
 10. Aluminium alloy according to claim 1, containing max. 0.2% w/w Cr.
 11. Aluminium alloy according to claim 1, containing 0.3 to 0.6% w/w Co.
 12. Aluminium alloy according to claim 1, containing 0.05 to 0.15% w/w Ti.
 13. Aluminium alloy according to claim 1, containing 0.1 to 0.4% w/w Zr.
 14. Aluminium alloy according to claim 1, containing 0.02 to 0.15% w/w V, and less than 60 ppm Be.
 15. Use of an aluminium alloy according to claim 1 for components subject to high thermal and mechanical stresses produced by pressure die casting, mould casting or sand casting.
 16. Use according to claim 15 for cylinder crankcases produced by the pressure die casting method in automotive engine construction.
 17. Use of an aluminium alloy according to claim 1 for safety components produced by the pressure die casting method in automotive construction.
 18. Aluminium alloy according to claim 1 containing 7.1 to 7.7% w/w Mg.
 19. Aluminium alloy according to claim 1 containing 3.1 to 3.6% w/w Si.
 20. Aluminium alloy according to claim 2 containing 3.0 to 3.7% w/w Si. 